A D-class amplifier is an amplifier that modulates a carrier signal by means of pulse width modulation, and, by switching the load current corresponding to the carrier signal, transmits a power-amplified signal to the load. It is also known as a digital amplifier. The D-class amplifier has an advantage that it can significantly lower the loss in power as compared to that of an A-class and B-class and other linear amplifiers (see Japanese Kokai Patent Application No. 2006-129366).
However, for devices that handle audio signals, when the amplifier is turned ON/OFF due to ON/OFF of the power source, due to transient operation of the circuit, the output signal varies drastically, and such variation in signal leads to unpleasant noise (popping noise). Consequently, especially for audio amplifiers, reduction in the noise generated in ON/OFF has become a topic to be addressed. In A-class and AB-class amplifiers having positive/negative outputs for driving speakers, several schemes have been proposed to reduce the popping noise when starting, such as a method in which positive/negative output voltages are gradually raised from the ground potential to an intermediate potential, and a method in which the popping noise at the start is reduced using a scheme of addition of a mute switch, relay or the like.
However, for a D-class amplifier, usually, even if the average value of the positive/negative output signals is zero, the load current is still switched corresponding to the carrier signal. Consequently, when started/stopped, discontinuous amplitude variation occurs between the non-switching state and the switching state, and rapid variation in the output signal caused by popping noise can readily occur, and this is undesired.
FIG. 6 is a waveform diagram illustrating variation in the output signal when a D-class amplifier is started/stopped.
When starting (left side in FIGS. 6(A), (B)), both the positive and negative outputs Pout1 and Pout2 initially become the ground potential (VL), and switching is stopped. On the other hand, after starting, a mute state forms (the right side in FIGS. 6(A), (B)), the positive and negative outputs Pout1 and Pout2 become switching waveforms in opposite phases and having a duty ratio of 50%. That is, in the start mode, as the state is changed from a state in which the switching is OFF to a state in which switching occurs at maximum amplitude, the amplitude of the carrier component of the output signal changes tremendously. In a stop operation, it is the same as above, and a large change takes place from a state in which switching is performed at maximum amplitude to a state in which switching is OFF (FIGS. 6(C), (D)). Here, because the outputs of the D-class amplifier are positive and negative outputs Pout1 and Pout2, in the state shown on the right side of FIGS. 6(A), (B), the average value of the output components is zero.
In order to suppress the variation in the output signals, a method has been proposed in which in the start operation, the amplitude of the switching waveform is gradually increased from zero to the maximum amplitude value. However, for a conventional D-class amplifier, because the power source voltage of the output stage becomes the maximum amplitude value of the switching waveform, the output voltage of the power source circuit for executing this method should be freely adjustable, leading to a complicated power source circuit and a fall in the power efficiency.
A general object of the present invention is to reduce or eliminate the problems of the prior art by providing a type of D-class amplifier that can suppress noise generated when the D-class amplification operation is turned ON/OFF.